This invention relates to a positioning apparatus (hereinafter a positioner) and more particularly to a positioner in a cellular base station that determines a position (positions) a portable terminal in a service area (cell) covered by the cellular base station.
Various methods of connection are employed in mobile communication systems such as portable telephone systems. Specifically, the methods of access can be generally classified into three methods, i.e., FDMA (frequency division multiple access) method, TDMA (time division multiple access) method and CDMA (code division multiple access) method. It has already been determined to use the CDMA method among those three methods for connection for IMT (international mobile telecommunication) 2000 which is the next generation portable telephone system because it employs a spread spectrum technique and therefore exhibits higher efficiency of frequency utilization compared to the TDMA method.
While there are many types of spread spectrum techniques (e.g., the direct spread (DS) method and frequency hopping (FH) method), this invention is applied to DS type spread spectrum communication (CDMA).
In such a CDMA type portable telephone system, it is desirable to position portable terminals. Such a need arises in consumer services and emergency car guiding systems. Consumer services are required by the IMT 2000 as an alternative to GPS navigation. Emergency call services are under evaluation and experiment in the United States as emergency car guiding systems.
Two methods described below have been used as systems for positioning portable terminals. One is a positioning system utilizing the GPS (global positioning system) method. In this positioning system, each portable terminal is equipped with a GPS receiver to allow the portable terminal to position itself, and the result of positioning at each portable terminal is transmitted to a base station to allow the base station to manage the information on the positions of the portable terminals.
The other is a positioning system utilizing a direction finder. In this positioning system, a plurality of base stations are provided with an azimuth detecting function, and a portable terminal is positioned based on a point at which azimuth lines defined by azimuth detection signals output by the base stations. The result of positioning may be transmitted from each base station to a portable terminal to allow its portable terminal to identify the position of itself.
However, the above-described positioning system utilizing the GPS method has problems as described below. The first problem is that it is subject to limitations during use in an urban area. The reason is that positioning requires reception signals from 3 or 4 CPS satellites, and it is difficult to identify locations that are blocked by buildings, roadside trees and the like. The second problem is that positioning accuracy is low. The reason is that positioning signals from GPS satellites open to public use are intentionally manipulated to provide low positioning accuracy in favor of the national interest of the United States.
The above-described positioning system utilizing a direction finder has problems as described below. The first problem is that it can not be used in existing cell configurations for portable telephones. The reason is that the positioning based on the output of a plurality of direction finders requires sufficient overlaps between the cells which does not currently exist. The second problem is that it is subject to limitations during use in an urban area. The reason is that it is necessary to maintain line of sight from several base stations to a portable terminal which is often difficult in an urban area.